1. Field of the Invention
This invention relates generally to a system for determining whether a compressor is unbalanced and, more particularly, to a system for determining whether a compressor for a fuel cell stack is unbalanced, where the system generates a DC analog signal indicative of the compressor balance from a sensor signal that includes an eigen-frequency signal and an unbalance frequency signal.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The compressor used to provide the cathode reactant gas flow is typically a high speed centrifugal air compressor that can operate up to about 80,000 RPMs. The compressor includes an electric motor that rotates a shaft coupled to a compressor wheel. If the compressor is unbalanced, i.e., the shaft has a slight oscillation, the high rotational speed of the shaft could cause damage to the compressor. Particularly, high speed compressors of this type typically use air bearings. If an unbalanced rotor shaft of the compressor contacts the air bearings, the compressor may be irreparably damaged. Therefore, it is desirable to detect small unbalances of the compressor rotor shaft for diagnostic purposes so that the compressor can be repaired before it is damaged.